Presently, silicon (Si) is the most commonly used material in the fabrication of solar cells, such solar cells being used for converting sunlight into electricity. Single and multi-junction p-n solar cells are used for this purpose, but none are efficient enough to significantly reduce the costs involved in the production and use of this technology. Consequently, competition from conventional sources of electricity precludes the widespread use of such solar cell technology.
Most electronic and optoelectronic devices require the formation of a junction. For example, a material of one conductivity type is placed in contact with a different material of the opposite conductivity type to form a heterojunction. Alternatively, one may pair differentially doped layers made of a single material type to generate a p-n junction (or homojunction). Abrupt band bending at a heterojunction due to a change in conductivity type and/or variations in band gap may lead to a high density of interface states that result in charge carrier recombination. Defects introduced at the junction during fabrication may further act as sites for charge carrier recombination that degrade device performance.
While the ideal thermodynamic efficiency of a solar converter is ˜85%, there is a loss in efficiency due to the fact that sub-bandgap energy photons in the solar spectrum are not absorbed. This loss alone, when applied with black-body radiation, limits the conversion efficiency of a single junction cell to about 44% (the so-called ultimate efficiency). Further taking into account the real solar spectrum normalized to a black body temperature, the temperature of the solar cell, the shape of the solar cell, the cell's refractive index, and the diode equations, Shockley and Queisser were able to show that the performance of a single junction cell was limited to just over 30 percent efficiency for an optimal cell with a bandgap of 1.45 electron volts (eV) and under 1 sun illumination, and just over 40% for maximum concentration (Shockley and Queisser, “Detailed Balance Limit of Efficiency of p-n Junction Solar Cells,” J. Appl. Phys., 1961, 32(3), pp. 510-519). More recent calculations have shown this “detailed balance limit efficiency” for a single junction to be 29 percent (Kerr et al., “Lifetime and efficiency of limits of crystalline silicon solar cells,” Proc. 29th IEEE Photovoltaic Specialists Conference, 2002, pp. 438-441). Additionally, recombination of photo-generated electrons and holes with trap states in the semiconductor crystal associated with point defects (interstitial impurities), metal clusters, line defects (dislocations), planar defects (stacking faults), and/or grain boundaries further reduces the efficiency. Although this latter reduction in efficiency can be overcome by using other materials with appropriate properties (particularly long diffusion lengths of the photo-generated carriers), this still does not bring this technology to a cost parity with more conventional sources of electricity.
Defect states due to structural imperfections or impurity atoms can reside on the surface and within the bulk of monocrystalline semiconductors. In addition, polycrystalline semiconductor materials are comprised of randomly-oriented crystal grains with grain boundaries, the grain boundaries inducing a large number of bulk and surface defect states. Because charge carriers can recombine at defect sites and are therefore lost as current carriers, defects typically adversely affect the operation or performance of electronic and/or optoelectronic devices such as solar cells. Accordingly, the surfaces of monocrystalline or polycrystalline semiconductor substrates are often passivated during device fabrication in order to minimize the negative effects of surface defects. One method for surface passivation is by forming a layer of intrinsic (undoped) amorphous semiconductor material on the monocrystalline or polycrystalline semiconductor substrate. This decreases the recombination of charge carriers at the substrate surface and improves the performance of the device.
The absorption capacity of the materials making up a PV device may also affect the efficiency of the cell. A p-i-n thin film solar cell having an i-type semiconductor absorber layer formed of a variable bandgap material, said i-layer being positioned between a p-type semiconductor layer and an n-type semiconductor layer has been described. See U.S. Pat. No. 5,252,142. A variable bandgap i-layer absorber provides for improved photoelectric conversion efficiency.
Multi-junction solar cells have been demonstrated to have improved efficiencies as well. The improved performance may be achieved by incorporating stacked junctions with differing band gaps to capture a broader area of the light spectrum. Such devices are typically constructed with stacked p-n junctions or stacked p-i-n junctions. Each set of junctions in this array is often referred to as a cell. A typical multi-junction solar cell includes of two or three cells stacked together. The optimal bandgaps and theoretical efficiencies for multi-junction solar cells as a function of number of cells in the stack has been analyzed theoretically by Marti and Araujo (A. Marti and G. L. Araujo, Sol. Ener. Mater. Sol. Cells, 1996, 43(2), pp. 203-222).
Nanostructures
Silicon nanowires have been described in p-n junction diode arrays (Peng et al., “Fabrication of large-Area Silicon Nanowire p-n Junction Diode Arrays,” Adv. Mater., 2004, vol. 16, pp. 73-76). Such arrays, however, were not configured for use in photovoltaic devices, nor was it suggested how such arrays might serve to increase the efficiency of solar cells.
Silicon nanostructures have been described in solar cell devices (Ji et al., “Silicon Nanostructures by Metal Induced Growth (MIG) for Solar Cell Emitters,” Proc. IEEE, 2002, pp. 1314-1317). In such devices, Si nanowires can be formed, embedded in microcrystalline Si thin films, by sputtering Si onto a nickel (Ni) pre-layer, the thickness of which determines whether the Si nanowires grow inside the film or not. However, such nanowires are not active photovoltaic (PV) elements; they merely serve in an anti-reflective capacity.
Solar cells comprising silicon nanostructures, where the nanostructures are active PV elements, have been described in commonly-assigned co-pending U.S. patent application Ser. No. 11/081,967, filed Mar. 16, 2005. In that particular Application, the charge separating junctions are largely contained within the nanostructures themselves, generally requiring doping changes during the synthesis of such nanostructures.
As a result of the foregoing, incorporating multi-junction cells over a nanostructured scaffold and fabrication methods that reduce the impact of defects may lead to solar cells with efficiencies on par with the more traditional sources of electricity. Thus, there is a continuing need to explore new configurations for PV devices. This is especially the case for nanostructured devices, which may benefit from enhanced light trapping and shorter paths for charge transport upon light absorption.